The mechanism by which mast cells are sensitized is the topic of this investigation. The site of fixation to mast cell receptors is on the Fc fragment of the antibody molecule. Hybrid antibody molecules produced by a hybridoma formed from anti-TNP IgG2b producing B cells and IgG1 producing myeloma cells will be used to examine whether the structure for fixation depends on tertiary or quaternary configurations of the Fc fragment. The helper T cells which collaborate with IgE and IgG1 antibody producing B cells will be examined. Experiments will be performed to decide if the same T cells help both IgE and IgG1 producing B cells or if there are two different helper T cells, one which collaborates with IgE and another which collaborates with IgG1 producing B cells. IgE antibody suppression has been shown in the SJL as well as other strains of mice. A method for IgE production has been worked out, and suppression of IgE antibody production has been obtained by injection of unprimed T cells. Another, more sophisticated adoptive transfer immunization schedule had to be devised in order to examine the mechanism of action of the unprimed T cells. In the adoptive transfer schedule, irradiated mice (600 R) are injected with a limited number of hapten primed B and carrier primed helper T cells as well as with T cells hyperprimed with a different carrier.